Turf grass (sod) is a living organism that must be handled properly to ensure its survival when it is removed from one location and transplanted to another. Sod is generally harvested using large machinery that cuts slabs of sod from the soil and stacks them on pallets. The speed at which a machine can stack slabs often determines the speed at which the machine can harvest sod.
FIG. 1 illustrates a portion of a sod harvesting machine 100 that includes a typical cutting head, conveyor system, and stacking system. The cutting head of FIG. 1 includes a first blade 111, a second blade 112, and a roller 113. Blade 111 is periodically lowered into the sod to make vertical cuts defining an initial width of the slabs. Blade 112 oscillates back and forth underneath the sod to sever the slab from the underlying soil. Roller 113 applies pressure to the sod as it is cut to facilitate the cutting of clean slabs.
Slabs cut by the cutting head are routed to conveyor 110a which lifts the slabs up to conveyor 110b. Once a sufficient number of slabs are positioned on conveyor 110b, stacking head 120 (or in some sod harvesters, multiple stacking heads) descends to the slabs, picks them up (e.g. via hooks), moves overtop a pallet, and drops the slabs on the pallet. This process continues until a pallet is filled.
A key factor that determines how quickly a sod harvester can operate is the rate at which the slabs can be removed from the conveyor and stacked on the pallet. To increase this rate, various approaches have been used. Of relevance to the present invention, some sod harvesting machines lift the conveyor towards the stacking head as opposed to dropping the stacking head to the conveyor. Lifting the conveyor increases the speed of slab removal because the stacking head is not required to move up and down. In other words, in such cases, the stacking head can quickly move back and forth from the conveyor to the pallet.
Various problems exist with current designs of conveyor lifting systems. Two of these current designs are shown in FIGS. 2A and 2B.
Some lifting systems push the slab up off of the secondary conveyor with a push through type of lifting tray which must lift and lower very quickly to be clear of the next slab coming up the conveyor. FIG. 2A, which includes reproductions of FIGS. 1A, 6A, and 6B of U.S. Pat. No. 8,118,154 to Tvetene, represents this type of lifting system. As best shown in the figure on the left, the conveyor is comprised of various thin belts 20a, 20b, and 20c with bump bed rails 35a-35d disposed therebetween. As shown in the figures on the right, each bump bed rail is raised up past the belts to lift a slab off the belts. The bump bed rails must raise and lower quickly which causes the hydraulic linear actuator seals used in such systems to wear out quickly and fail leading to frequent replacement and high maintenance costs.
Some lifting systems lift the entire conveyor. FIG. 2B, which is a reproduction of FIG. 1 of U.S. Pat. No. 8,336,638 to Brouwer, et al., shows a lifting systems that lifts the entire conveyor up to the capture position and then returns to the rest position. As shown, a bed frame 140 that houses the conveyor is lifted and lowered by piston and cylinder sets 142. This operation is slow and requires that the slabs be spaced far apart. This adversely affects the productivity of the machine.
In each of these different types of systems, because of the inertia of the lift tray, lift linkage, and slab, large forces are required of the actuators in high speed lifts. Thus in hydraulic systems, the actuators must be relatively large with correspondingly high peak fluid flows. Unless a high pressure fluid accumulator is used, the peak flow will drop the system pressure in other parts of the system disturbing the performance of other hydraulic functions. A high pressure accumulator is an additional manufacturing and maintenance cost.